System and method for non-sinusoidal current waveform excitation of electrical generators

ABSTRACT

An electrical generator includes a stator having fractional-slot concentrated windings and a rotor having field windings. A drive is provided having a circuit to control current flow to the field windings and a controller to input an initial DC field current demand to the circuit to cause the circuit to output an initial DC field current representative of a DC field current demand that would cause an electrical generator having sinusoidal stator windings to output a desired AC power. The controller receives feedback on the magnetic field generated by the initial DC field current, isolates an ideal fundamental component of the magnetic field based on the feedback and to generate a modified DC field current demand, and inputs the modified DC field current demand to the circuit, thereby causing the circuit to output an instantaneous non-sinusoidal current to the field windings to generate a sinusoidal rotating air gap magnetic field.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of, and claims priority to, U.S.patent application Ser. No. 12/826,076 filed on Jun. 29, 2010, thedisclosure of which is incorporated herein in its entirety.

BACKGROUND OF THE INVENTION

The invention relates generally to electrical generators and, moreparticularly, to a control scheme for exciting an electrical generatorhaving fractional-slot concentrated windings and rotor field windings.

The usage of electrical machines in various industries has continued tobecome more prevalent in numerous industrial, commercial, andtransportation industries over time. There has been tremendous progressand great achievements in the field of power electronics and controltechniques for such electrical machines, resulting in increased energysavings and control flexibility. Providing for such achievements hasbeen the continued progress in computer technology that has resultedfrom digital technology. Digital technology has lead to very significantreductions in the size and cost of computers, allowing them tosuccessfully replace old, bulky, and relatively expensive mechanicalsystems.

While the capability of digitally enhanced control systems and computershas progressed, the structure of the electrical machines used with suchcontrol systems has, for the most part, remained unchanged. For example,the large majority of fixed speed electrical generators, such as thoseused in power stations, are designed using distributed sinusoidalwindings on the stator and a DC field or permanent magnets on the rotor.As shown in FIG. 1, a prior art electrical generator 2 may be equippedwith integral-slot distributed stator windings 4 and permanent magnets 6on the rotor 8. As an example, FIG. 1 illustrates a 24-slot, overlappingdistributed arrangement of stator windings 4. In operation, thepermanent magnets 6 of generator 2 create the magnetic field in the airgap between rotor 8 and stator windings 4, which rotates the rotor 8 andgenerates electrical energy in the stator windings 4.

Construction of electrical generators in accordance with the structureof generator 2 illustrated in FIG. 1, that implement distributedsinusoidal windings 4, are however subject to drawbacks in performanceand costs associated therewith. For example, electrical generators thatimplement distributed sinusoidal windings suffer from a decreasedefficiency due to electrical losses in the end windings. The end windinglength contributes to increased resistance, thereby resulting in higherOhmic losses that decrease the efficiency of the generator. The endwinding length also requires implementation of a complex cooling system(e.g., liquid hydrogen cooling system), which leads to increased coolingcost in the electrical generator. Furthermore, the permanent magnetslimit power density, energy efficiency, operating temperature, lifecycle, and reliability of the electrical generator.

In addition to increased operating costs, electrical generators such asshown in FIG. 1 that implement distributed sinusoidal windings andpermanent magnets are also more costly to construct. For example, suchelectrical generators often include expensive thin stator laminationsthat are expensive to construct. Furthermore, the permanent magnets onthe rotor used to create the air gap magnetic field are expensivecompared to generators incorporating electromagnets or field windings.

Therefore, it would be desirable to design an electrical generator thatcan employ non-sinusoidal stator windings so as to reduce costsassociated with production and operation thereof. It is further desiredthat a control scheme be provided for controlling electrical generatorsthat employ non-sinusoidal stator windings that suppresses the effect ofthe additional harmonic components typically associated withnon-sinusoidal windings, so as to minimize harmonics and maintain highpower density and high efficiency in the generator.

BRIEF DESCRIPTION OF THE INVENTION

Embodiments of the invention overcome the aforementioned drawbacks byproviding a system and method for exciting an electrical generatorhaving fractional-slot concentrated windings and rotor field windings byway of an instantaneous, non-sinusoidal current.

In accordance with one aspect of the invention, an electrical generatorincludes a stator having a plurality of fractional-slot concentratedwindings, a rotor positioned within the stator to rotate relativethereto and having field windings electrically coupled theretoconfigured to generate a rotating magnetic field in an air gap betweenthe stator and the rotor responsive to a current applied thereto, and adrive having an input connectable to a power source and an outputconnectable to the field windings. The drive further includes a circuitconfigured to control current flow to the field windings and acontroller connected to the circuit and programmed to input an initialDC field current demand to the circuit to cause the circuit to output aninitial DC field current, with the initial DC field current demand beingrepresentative of a DC field current demand that would cause anelectrical generator having sinusoidal stator windings to output adesired AC power. The controller is further programmed to receivefeedback on the rotating magnetic field generated by the initial DCfield current, determine and isolate an ideal fundamental component ofthe rotating magnetic field based on the feedback, generate a modifiedDC field current demand based on the ideal fundamental component, andinput the modified DC field current demand to the circuit, therebycausing the circuit to output an instantaneous non-sinusoidal current tothe field windings to generate a sinusoidal rotating air gap magneticfield.

In accordance with another aspect of the invention, a method forgenerating AC power in an electrical generator having a stator having aplurality of fractional-slot concentrated windings and a rotor having aplurality of field windings is provided, that includes inputting a testDC field current demand to an inverter that is representative of a DCfield current demand that would cause an electrical generator havingsinusoidal stator windings to output a desired AC power and generatingan initial DC field current in the inverter in response to the test DCfield current demand, with the initial DC field current being output tothe plurality of field windings on the rotor to generate a test rotatingmagnetic field between the rotor and the stator. The method alsoincludes determining a fundamental component and harmonic components ofthe rotating magnetic field, determining an ideal fundamental componentfor the rotating magnetic field from the test DC field current demandand the fundamental component, and determining a desired currentwaveform based on the ideal fundamental component. The method furtherincludes generating a modified DC field current demand based on thedesired current waveform and inputting the modified DC field currentdemand to the inverter, thereby causing the inverter to output anon-sinusoidal AC current waveform to the plurality of field windings onthe rotor to generate a sinusoidal rotating magnetic field, therebygenerating AC power in the electrical generator.

In accordance with yet another aspect of the invention, an electricalgenerator includes a stator having a plurality of non-sinusoidalconcentrated windings, a rotor positioned within the stator to rotaterelative thereto and having field windings configured to generate arotating magnetic field in an air gap between the stator and the rotorresponsive to a current applied thereto, and a drive to control currentflow from a power source to the field windings. The drive is configuredto provide an initial input current to the rotor based on a firstcurrent demand that is representative of a DC field current demand thatwould cause an electrical generator having sinusoidal stator windings tooutput a desired AC power and receive feedback on the rotating magneticfield generated by the initial input current. The drive is furtherconfigured to generate a second current demand based on the feedback andprovide an instantaneous modified input current to the field windingsbased on the second current demand so as to generate a sinusoidalrotating magnetic field in the air gap and generate AC power in theelectrical generator, wherein the instantaneous modified input currentcomprises a non-sinusoidal current waveform.

Various other features and advantages will be made apparent from thefollowing detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate preferred embodiments presently contemplated forcarrying out the invention.

In the drawings:

FIG. 1 is a schematic diagram of a prior art stator windingconfiguration for an electrical generator.

FIG. 2 a schematic of an AC drive according to an embodiment of theinvention.

FIG. 3 is a schematic diagram of a stator winding configuration andfield winding arrangement for an electrical generator according to anembodiment of the invention.

FIG. 4 is a flow diagram of a controller implemented technique forcontrolling an AC drive according to an embodiment of the invention.

FIG. 5 is a block schematic diagram of an apparatus for producingtractive effort incorporating a drive and electrical generator accordingto an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiments of the invention are directed to electrical generatorshaving non-sinusoidal concentrated stator windings and to a controlscheme for providing current to field windings on the rotor to generatea sinusoidal air gap magnetic field between the rotor and the stator.The control scheme processes an initial DC field current demand appliedto an inverter/electronic processing circuit in order to generateinstantaneous non-sinusoidal current demands that, when applied to thefield windings, will produce rotating air gap magnetic fields with onlyfundamental components and eliminate all field harmonics, thus resultingin the best energy conversion from the rotor to the stator, i.e. highoutput power at high efficiency.

Embodiments of the invention are directed to electrical generators,including variable and fixed speed electrical generators, and to acontrol scheme for operating the electrical generators. The generalstructure of an AC drive 10 is shown in FIG. 2 according to oneembodiment of the invention. The drive 10 may be configured, forexample, as an adjustable speed drive (ASD) designed to receive a threephase AC power input, rectify the AC input, and perform a DC/ACconversion of the rectified segment into a three-phase alternatingvoltage of variable frequency and amplitude that is supplied to anelectrical machine, such as an electrical generator. According toalternate embodiments, it is recognized that drive 10 may be designed toreceive a DC power input and perform a DC/AC conversion of the DC powerinto a multi-phase alternating voltage of variable frequency andamplitude that is supplied to an electrical machine. In a preferredembodiment, the ASD operates according to an exemplary volts-per-hertzcharacteristic. In this regard, the drive provides voltage regulation of±1% in steady state with less than 3% total harmonic distortion, ±0.1 Hzin output frequency, and fast dynamic step load response over a fullload range.

According to one embodiment, a three-phase AC input 12 a-12 c is fed toa three-phase rectifier bridge 14. The input line impedances are equalin all three phases. The rectifier bridge 14 converts the AC power inputto a DC power such that a DC bus voltage is present between therectifier bridge 14 and a switch array 16. The bus voltage is smoothedby a DC bus capacitor bank 18. The switch array 16 is comprised of aseries of IGBT switches 20 and anti-parallel diodes 22 that collectivelyform an inverter or chopper circuit 24. The inverter 24 synthesizes ACvoltage waveforms for delivery to an electrical machine, such as anelectrical generator 26 according to current demands generated by adrive controller 28, as will be explained in greater detail below. Thecontroller 28 interfaces to the inverter 24 via current demand signalsand sensing of the DC bus voltage and pole currents (by way a voltagesensor 34 for example) such that changes in DC bus voltage can besensed. These voltage changes can be interpreted as transient loadconditions and are used in the generation/input of instantaneous currentdemands to inverter 24, such that near steady-state load conditions aremaintained.

According to the embodiment shown in FIG. 2, electrical generator 26 isin the form of a variable speed electrical generator that implementsinverter 24 to provide for variable speed operation. However, it is alsorecognized that embodiments of the invention can also be directed to afixed speed electrical generator (e.g., an alternator), in which case asimplified drive 10 could be implemented in which inverter 24 isreplaced by a more simplified electronic processing circuit (not shown)that synthesizes AC voltage waveforms for delivery to electricalgenerator 26 according to current demands generated by drive controller28.

According to an embodiment of the invention, electrical generator 26 hasa construction such as shown in FIG. 3, for example. As shown in FIG. 3,electrical generator 26 includes therein a stator 36 having a statorcore 40 and windings 42 wound on the stator core 40. The stator core 40has a core main body 44 formed by stacking a large number ofannular-shaped thin plates made of electromagnetic steel and insulators(not shown) provided on axial end surfaces of the core main body. Thestator core 40 is provided with a plurality of teeth 46 at apredetermined pitch along a circumferential direction thereof. Accordingto an exemplary embodiment, windings 42 are wound on the respectiveteeth 46, and thus are in the form of fractional-slot concentratedwindings or “tooth windings.” Slots 48 are formed between adjacent teeth46 along the circumferential direction. As shown in FIG. 3, oneembodiment of stator 36 includes six slots 48, with a non-overlappingconcentrated arrangement of windings 42 wound about all teeth 46,according to an embodiment of the invention. It is recognized thatelectrical generators 26 including other arrangements of concentratedwindings are envisioned as being usable with embodiments of the presentinvention, and thus the winding arrangement of FIG. 3 is merelyexemplary.

As further shown in FIG. 3, a rotor 50 is rotatably fitted in the stator36. The rotor 50 has coils wound thereon to form four-pole fieldwindings 52 that when supplied with an excitation current, such as thenon-sinusoidal current waveform explained in greater detail below, willexcite the DC magnetic field on the rotor 50 so as to generate arotating magnetic field in an air gap 54 between the rotor 50 and thestator 36. The field windings 52 can receive such an excitation currentfrom inverter 24 (FIG. 2), for example, according to current demandsgenerated by drive controller 28. Thus, upon a supplying of current tofield windings 52, the rotating magnetic field. generated in air gap 54between the rotor and the stator causes AC power to be generated instator 36, such that electrical generator 26 generates AC power.

Referring now to FIG. 4, and with continued reference to FIGS. 2 and 3,a block diagram is shown representative of a control scheme 56 foroperating drive 10 that is implemented, for example, by controller 28.The control scheme 56 of FIG. 4 performs electronic processing details(EPD) used together with an electrical generator 26 havingfractional-slot concentrated stator windings 42 and rotor field windings52 to achieve high power density, high efficiency, and reduced cost ofboth the electrical generator 26 and the inverter 24. That is, controlscheme 56 is implemented in order to generate instantaneousnon-sinusoidal current demands that will excite the DC magnetic field onthe rotor 50 and generate rotating air gap fields with only fundamentalcomponents and eliminate all field harmonics, thus resulting in the bestenergy conversion from the rotor 50 to the stator 36, i.e. high outputpower at high efficiency.

Initially, BLOCK 58 of control scheme 56 performs a selective“synchronization time function” operation on a received first inputcurrent 60 and received second input current 62. The first input current60 is an initial or test current input that is generated in response toan initial DC field current demand, and thus is termed an initial DCfield current. The initial DC field current demand is representative ofa current demand needed to generate the DC magnetic field on the rotorif a perfect sinusoidal winding were used on the stator of electricalgenerator 26. In an initial iteration, or test/setup run, second inputcurrent 62 is absent.

The first input 60 passes through BLOCK 58 unaffected (i.e., no timesynchronization performed on first input 60) responsive to the initialDC field current demand. The first input 60 is thus received at BLOCK64, with BLOCK 64 functioning to adjust an amplitude of the DC fieldcurrent according to, for example, a stored table of current demandvalues and appropriate adjusting factors. For example, if first input 60is representative of a high DC field current demand, the amplitude isadjusted so as to avoid producing high losses in the stator 36.Conversely, if first input 60 is representative of a very low DC fieldcurrent demand, the demanded DC field current will be adjustedappropriately (i.e., increased) so as to produce a rotating air gapmagnetic field that can be easily detected, as will be explained below.The adjustment factor added by BLOCK 64 is then stored for later use asa reference loop to produce the real instantaneous current, (i.e.,second current loop 62), which is used to produce the clean demandedsinusoidal output power from electrical generator 26, as will beexplained below.

When applied to the adjusted DC field current added at BLOCK 66 willproduce a rotating air gap magnetic field between the rotor 50 and thestator 36, with the air gap magnetic field including unwanted harmoniccomponents due to the concentrated/concentric windings 42 of the stator.The rotating air gap magnetic field is then detected by using hightemperature Hall probes 66 integrated into electrical machine 26, forexample, with the number of Hall probes 66 needed depending on thestator inside diameter and the resolution needed for downstream accuratesignal processing. Alternatively, search coils (not shown) locatedpreferably at the center of the stator 36 could be implemented, with thesearch coils being added during winding of the stator coils 42 and beingkept inside the stator slots 48.

The output of the search coils/Hall probes 66 is transmitted to BLOCK 68and is received thereby (i.e., received by controller 28) as feedback ona strength of the rotating air gap magnetic field. A fast Fouriertransform (FFT) is performed on the air gap magnetic field feedback atBLOCK 68 to determine/analyze the fundamental component and the harmoniccomponents of the air gap rotating field. That is, instantaneous valuesof the fundamental component and the harmonic components of the air gaprotating field are determined.

Values for the instantaneous fundamental component and the instantaneousharmonic components of the air gap rotating field determined in BLOCK 68are passed to BLOCK 70, which acts to eliminate the harmonic componentsof the air gap magnetic field. The fundamental component of the air gapmagnetic field is thus isolated and is subsequently passed to BLOCK 72.As shown in FIG. 4, the isolated instantaneous fundamental component ofthe air gap magnetic field is input to BLOCK 72 along with the firstinput 60 (i.e., the initial DC field current). A lookup table is storedin BLOCK 72 that has stored therein a plurality of DC field currents andthe ideal fundamental component of a rotating magnetic field generatedfrom each of the plurality of DC field currents. The “ideal” fundamentalcomponent of the rotating magnetic field associated with each demandedDC field current is defined in the lookup table as the highestfundamental component generated by input of the demanded DC fieldcurrent to an electrical generator having sinusoidal windings.

The isolated instantaneous fundamental component of the air gap magneticfield and the initial DC field current of the first input 60 areanalyzed/compared to the lookup table in BLOCK 72. More specifically,the instantaneous fundamental component of the air gap magnetic fieldand the initial DC field current are analyzed with respect to the lookuptable to determine what DC field current need be applied to anelectrical generator having sinusoidal windings in order to generate theinstantaneous fundamental component of the air gap magnetic field. Basedon this determination, a correction is applied to the instantaneousfundamental component of the rotating magnetic field, such that theideal fundamental component for the needed DC field current is realized.

Referring still to FIG. 4, upon determination of the ideal fundamentalcomponent of the rotating magnetic field, the ideal fundamentalcomponent is input to BLOCK 74. Also in BLOCK 74, the adjustment factorpreviously applied to the initial DC field current in BLOCK 64 isremoved, by having an input from BLOCK 64 to BLOCK 74 to cancel theadjustment done earlier. That is, BLOCK 74 has a table which takes theamplitude of the fundamental air gap field produced from BLOCK 72 anddetermines a DC field current ideally needed to produce it. Thisdetermined DC field current is multiplied by the inverse of thecorrection factor from BLOCK 64 and compared to the initial DC fieldcurrent to make sure that they are identical. This fundamental air gapmagnetic field is the best representation of the fundamental air gapfield at that time for the original DC field current (which containsboth time and space harmonics) as if the electrical generator 26 had aperfectly sinusoidal stator winding.

A “true” fundamental air gap signal is thus output from BLOCK 74 andreceived by BLOCK 76. At BLOCK 76, a Laplace transform is performed onthe signal from BLOCK 74. Next, BLOCK 78 represents the Laplace transferfunction of the concentrated winding in the electrical machine. Thetransfer function of BLOCK 78 is obtained between the first input 60(i.e., the amplitude of the initial DC field current) to the electricalgenerator 26 and the rotating magnetic field as measured by the searchcoils/Hall probes (i.e., the fundamental of the rotating magneticfield). This is measured over the full speed range of the electricalgenerator 26 using standard small signal perturbation techniques, asknown in the control industry.

The output of BLOCK 76, which is the Laplace transfer of theinstantaneous air gap magnetic field, is considered to be the input ofBLOCK 78, which is the transfer function of the fractional-slotconcentrated winding. Next, at BLOCK 80, the inverse Laplace transformis applied to the output of BLOCK 78 to re-construct the exactinstantaneous low voltage current waveform that, when applied to theinverter, will produce the desired instantaneous current. A desiredcurrent waveform for generating the ideal fundamental component of therotating magnetic field is thus determined from BLOCKS 76, 78, and 80.Based on the desired current waveform, a modified DC field currentdemand is generated at BLOCK 82 that will produce the desired currentwaveform when applied to the inverter 24, so as to produce the requiredfundamental air gap field.

At BLOCK 58, the first input current 60 is zeroed and a synchronizationtime function operation is performed on the second current 62 accordingto the modified DC field current demand, so as to adjust the timing ofthe second input current. Application of the modified DC field currentdemand to inverter 24 generates a modified input current (i.e., secondcurrent 62), which is output from inverter 24 in the form of a highpower, instantaneous non-sinusoidal current. The adjusted (i.e.,instantaneous) non-sinusoidal current is applied to the field windings52 of the rotor 50 to produce a sinusoidal air gap magnetic field thatproduces a high output power with minimum losses, as there are noharmonics in the air gap field despite the fact that aconcentrated/concentric winding 42 (i.e., non-sinusoidal winding) isused on the stator 36.

Referring now to FIG. 5, a drive 84 implementing a controlscheme/electronic processing as described in FIG. 4 is shown asincorporated into a hybrid electric vehicle (HEV) AC propulsion system86 that produces tractive effort and implements regenerative braking,according to an embodiment of the invention. Propulsion system 86includes drive 84, an energy source 88, and an electrical machine 90configured to selectively operate both as an electric motor andelectrical generator, with the electrical machine having fractional-slotconcentrated windings, or tooth windings.

In producing tractive effort from system 86, energy source 88 generatesa high DC voltage 92 and drive 84 generates a multi-phase motor voltage94 from high DC voltage 92, with electrical machine 90 producingtractive effort from motor voltage 94. In the embodiment of FIG. 5,energy source 88 is configured as a hybrid-electric energy source thatcomprises a heat engine 96, an alternator 98, a rectifier 100, atraction/energy battery 102, and a traction boost converter 104.Traction boost converter 104 is sometimes referred to as bi-directionalDC-DC converter, or a bi-directional boost/buck converter that functionsto decouple the voltage between the input and the output of the devicewhile efficiently transferring power. In operation to produce tractiveeffort, heat engine 96 generates mechanical power 106 by burning a fuel.Alternator 98 generates an alternating voltage 108 from mechanical power106 and rectifier 100 then rectifies alternating voltage 108 to producea low DC voltage 110. Traction battery 102 stores and delivers energyderived from low DC voltage 110 and traction boost converter 104 boostslow DC voltage 110 to produce high DC voltage 92. As used herein inreference to DC voltages, “low” and “high” are relative terms only andimply no particular absolute voltage levels. The high DC voltage 92 istransferred to drive 84, which includes therein a converter/inverter 112that receives high DC voltage 92 and, responsive thereto, generates amultiphase AC motor voltage 94 from high DC voltage 92 that is providedto electrical machine to produce tractive effort.

Additionally, converter/inverter 112 is configured to generate high DCvoltage 92 from multi-phase motor voltage 94 during a regenerativebraking operation. That is, during a regenerative braking operation, theelectrical machine 90 is caused to operate with a torque that isopposite polarity of the torque that produces acceleration (reversedtorque as opposed to a torque that produces traction power) therebycausing it to decelerate or slow the vehicle's wheels. While running ina regenerative braking mode, the electrical machine 90 acts as anelectrical generator, producing a multi-phase voltage 94 that is fedthrough traction inverter/converter 112 into the traction battery 102.Specifically, the high DC voltage 92 that is produced from motormulti-phase voltage 94 flows from the high voltage side 92 of thebi-directional DC-DC converter 104 to the lower voltage side 110 of thebi-directional DC-DC converter 104 through a “buck” mode of operation.

For controlling electrical machine 90 during regenerative braking tocause the electrical machine to operate as a generator, drive 84generates an initial DC field current demand (i.e., a first input). Asset forth in detail with respect to FIG. 4, drive 84 (i.e., a controllerin motor drive 84) transmits the initial DC field current demand totraction inverter 112 to produce an initial current for transmission toa rotor (not shown) of electrical machine 90. Electrical generator 90generates a rotating air gap magnetic field responsive to the initial DCfield current demand, from which drive 84 determines fundamental andharmonic components. The harmonic components of the air gap magneticfield are eliminated and motor drive 84 applies a correction to thefundamental component to generate an ideal fundamental component. Aninstantaneous current needed to generate the ideal fundamental componentin electrical generator 90 and an exact instantaneous low voltagecurrent waveform that when applied to the converter 112 will produce theneeded instantaneous current, are determined by drive 84. Based on thedesired current waveform, drive 84 generates an instantaneousnon-sinusoidal DC field current demand that will produce the desiredcurrent waveform when applied to the converter/inverter 112. Based onthe instantaneous non-sinusoidal DC field current demand from drive 84,converter/inverter 112 therefore generates and transmits a modified orsecond input 114 to electrical machine 90 having a desirednon-sinusoidal current waveform.

While drive 84 and an accompanying electrical machine 90 are describedin FIG. 5 as being incorporated into a HEV system 86 that producestractive effort and electrical power via regenerative braking, it isrecognized that a drive configured to implement a technique for excitingan electrical generator with instantaneous non-sinusoidal currentwaveforms is also applicable to other varied types of electricalmachines. Thus, embodiments of the invention directed to drives anddrive controllers are applicable to electrical machines in numerousindustrial, commercial, and transportation industries.

A technical contribution for the disclosed method and apparatus is thatit provides for a controller implemented technique for exciting anelectrical generator having concentrated stator windings by way ofinstantaneous non-sinusoidal current waveforms sent to rotor fieldwindings. A control scheme is implemented that processes an initial DCfield current demand applied to the inverter in order to generateinstantaneous non-sinusoidal DC field current demands that will producerotating air gap fields with only fundamental components and eliminateall field harmonics, thus resulting in the best energy conversion fromthe rotor to the stator, i.e. high power output at high efficiency.

Therefore, according to one embodiment of the invention, an electricalgenerator includes a stator having a plurality of fractional-slotconcentrated windings, a rotor positioned within the stator to rotaterelative thereto and having field windings electrically coupled theretoconfigured to generate a rotating magnetic field in an air gap betweenthe stator and the rotor responsive to a current applied thereto, and adrive having an input connectable to a power source and an outputconnectable to the field windings. The drive further includes a circuitconfigured to control current flow to the field windings and acontroller connected to the circuit and programmed to input an initialDC field current demand to the circuit to cause the circuit to output aninitial DC field current, with the initial DC field current demand beingrepresentative of a DC field current demand that would cause anelectrical generator having sinusoidal stator windings to output adesired AC power. The controller is further programmed to receivefeedback on the rotating magnetic field generated by the initial DCfield current, determine and isolate an ideal fundamental component ofthe rotating magnetic field based on the feedback, generate a modifiedDC field current demand based on the ideal fundamental component, andinput the modified DC field current demand to the circuit, therebycausing the circuit to output an instantaneous non-sinusoidal current tothe field windings to generate a sinusoidal rotating air gap magneticfield.

According to another embodiment of the invention, a method forgenerating AC power in an electrical generator having a stator having aplurality of fractional-slot concentrated windings and a rotor having aplurality of field windings is provided, that includes inputting a testDC field current demand to an inverter that is representative of a DCfield current demand that would cause an electrical generator havingsinusoidal stator windings to output a desired AC power and generatingan initial DC field current in the inverter in response to the test DCfield current demand, with the initial DC field current being output tothe plurality of field windings on the rotor to generate a test rotatingmagnetic field between the rotor and the stator. The method alsoincludes determining a fundamental component and harmonic components ofthe rotating magnetic field, determining an ideal fundamental componentfor the rotating magnetic field from the test DC field current demandand the fundamental component, and determining a desired currentwaveform based on the ideal fundamental component. The method furtherincludes generating a modified DC field current demand based on thedesired current waveform and inputting the modified DC field currentdemand to the inverter, thereby causing the inverter to output anon-sinusoidal AC current waveform to the plurality of field windings onthe rotor to generate a sinusoidal rotating magnetic field, therebygenerating AC power in the electrical generator.

According to yet another embodiment of the invention, an electricalgenerator includes a stator having a plurality of non-sinusoidalconcentrated windings, a rotor positioned within the stator to rotaterelative thereto and having field windings configured to generate arotating magnetic field in an air gap between the stator and the rotorresponsive to a current applied thereto, and a drive to control currentflow from a power source to the field windings. The drive is configuredto provide an initial input current to the rotor based on a firstcurrent demand that is representative of a DC field current demand thatwould cause an electrical generator having sinusoidal stator windings tooutput a desired AC power and receive feedback on the rotating magneticfield generated by the initial input current. The drive is furtherconfigured to generate a second current demand based on the feedback andprovide an instantaneous modified input current to the field windingsbased on the second current demand so as to generate a sinusoidalrotating magnetic field in the air gap and generate AC power in theelectrical generator, wherein the instantaneous modified input currentcomprises a non-sinusoidal current waveform.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

What is claimed is:
 1. A method for generating AC power in an electricalgenerator including a stator having a plurality of fractional-slotconcentrated windings and a rotor having a plurality of field windings,the method comprising: inputting a test DC field current demand to aninverter; generating an initial DC field current in the inverter inresponse to the test DC field current demand, the initial DC fieldcurrent being output to the plurality of field windings on the rotor togenerate a test rotating magnetic field between the rotor and thestator; detecting characteristics of the test rotating magnetic fieldgenerated by the initial DC field current; determining a desired currentwaveform based on the detected characteristics of the test rotatingmagnetic field; generating a modified DC field current demand, based onthe desired current waveform, to produce the desired current waveform;and inputting the desired current waveform to the inverter, therebycausing the inverter to output a non-sinusoidal AC current waveform tothe plurality of field windings on the rotor to generate a sinusoidalrotating magnetic field, thereby generating AC power in the electricalgenerator.
 2. The method of claim 1 further comprising: determining afundamental component and harmonic components of the test rotatingmagnetic field; determining an ideal fundamental component for the testrotating magnetic field from the test DC field current demand and thefundamental component; and determining the desired current waveformbased on the ideal fundamental component.
 3. The method of claim 2wherein determining the fundamental component and harmonic componentscomprises: detecting the test rotating magnetic field via one of asearch coil and a plurality of Hall effect sensors; and applying a fastFourier transform (FFT) to the detected test rotating magnetic field toidentify the fundamental component and harmonic components.
 4. Themethod of claim 2 wherein determining the ideal fundamental component ofthe test rotating magnetic field comprises accessing a lookup tablehaving stored therein each of a plurality of DC field current demandsand ideal fundamental components of a rotating magnetic field generatedfrom each of the plurality of DC field current demands, each of theideal fundamental components representing a highest fundamentalcomponent resulting from input of a DC field current demand to anelectrical generator having sinusoidal stator windings.
 5. The method ofclaim 2 further comprising performing a Laplace transfer function of thetest rotating magnetic field, the Laplace transfer function of the testrotating magnetic field derived from the initial DC field current andthe ideal fundamental component of the rotating magnetic field.
 6. Themethod of claim 1 wherein the modified DC field current demand adjusts atiming of an input current from a power source to produce the desiredcurrent waveform, thereby causing the inverter to output anon-sinusoidal AC current waveform to the plurality of field windings soas to generate a sinusoidal rotating magnetic field having the idealfundamental component.
 7. The method of claim 1 wherein the test DCfield current demand is representative of a DC field current demand thatwould cause an electrical generator having sinusoidal stator windings tooutput a desired AC power.
 8. A drive for controlling operation of anelectrical generator having a stator with a plurality of fractional-slotconcentrated windings and a rotor positioned within the stator andconfigured to rotate relative thereto, the rotor having field windingselectrically coupled thereto configured to generate a rotating magneticfield in an air gap between the stator and the rotor responsive to acurrent applied thereto, the drive comprising: an inverter configured tocontrol current flow to the field windings; and a controller connectedto the inverter and programmed to: input a test DC field current demandto an inverter; generate an initial DC field current in the inverter inresponse to the test DC field current demand, the initial DC fieldcurrent being output to the plurality of field windings on the rotor togenerate a test rotating magnetic field between the rotor and thestator; detect characteristics of the test rotating magnetic fieldgenerated by the initial DC field current; determine a desired currentwaveform based on the detected characteristics of the test rotatingmagnetic field; generate a modified DC field current demand, based onthe desired current waveform, to produce the desired current waveform;and input the desired current waveform to the inverter, thereby causingthe inverter to output a non-sinusoidal AC current waveform to theplurality of field windings on the rotor to generate a sinusoidalrotating magnetic field, thereby generating AC power in the electricalgenerator
 9. The drive of claim 8 wherein the controller is furtherprogrammed to: determine a fundamental component and harmonic componentsof the rotating magnetic field; determine an ideal fundamental componentfor the rotating magnetic field from the test DC field current demandand the fundamental component; and determine the desired currentwaveform based on the ideal fundamental component.
 10. The drive ofclaim 9 wherein, in determining the fundamental component and harmoniccomponents, the controller is further programmed to: detect the testrotating magnetic field via one of a search coil and a plurality of Halleffect sensors; and apply a fast Fourier transform (FFT) to the detectedtest rotating magnetic field to identify the fundamental component andharmonic components.
 11. The drive of claim 9 wherein, in determiningthe fundamental component and harmonic components, the controller isfurther programmed to access a lookup table having stored therein eachof a plurality of DC field current demands and ideal fundamentalcomponents of a rotating magnetic field generated from each of theplurality of DC field current demands, each of the ideal fundamentalcomponents representing a highest fundamental component resulting frominput of a DC field current demand to an electrical generator havingsinusoidal stator windings.
 12. The drive of claim 8 wherein themodified DC field current demand adjusts a timing of an input currentfrom a power source to produce the desired current waveform, therebycausing the inverter to output a non-sinusoidal AC current waveform tothe plurality of field windings so as to generate a sinusoidal rotatingmagnetic field having the ideal fundamental component.
 13. The drive ofclaim 8 wherein the test DC field current demand is representative of aDC field current demand that would cause an electrical generator havingsinusoidal stator windings to output a desired AC power.
 14. The driveof claim 8 wherein the controller is further programmed to perform aLaplace transfer function of the test rotating magnetic field, theLaplace transfer function of the test rotating magnetic field derivedfrom the initial DC field current and the ideal fundamental component ofthe rotating magnetic field.
 15. A method for generating AC power in anelectrical generator including a stator having a plurality offractional-slot concentrated windings and a rotor having a plurality offield windings, the method comprising: inputting a test DC field currentdemand to an inverter, the test DC field current demand beingrepresentative of a DC field current demand that would cause anelectrical generator having sinusoidal stator windings to output adesired AC power; generating an initial DC field current in the inverterin response to the test DC field current demand, the initial DC fieldcurrent being output to the plurality of field windings on the rotor togenerate a test rotating magnetic field between the rotor and thestator; determining a fundamental component and harmonic components ofthe rotating magnetic field; determining an ideal fundamental componentfor the rotating magnetic field from the test DC field current demandand the fundamental component; determining a desired current waveformbased on the ideal fundamental component; generating a modified DC fieldcurrent demand based on the desired current waveform; and inputting themodified DC field current demand to the inverter, thereby causing theinverter to output a non-sinusoidal AC current waveform to the pluralityof field windings on the rotor to generate a sinusoidal rotatingmagnetic field, thereby generating AC power in the electrical generator.16. The method of claim 15 further comprising modifying an amplitude ofthe initial DC field current to generate the test rotating magneticfield in the electrical generator.